Zinc oxide thin films are well known for use as a piezoelectric material for such devices as a surface acoustic wave device, a tuning fork vibrator or a tuning bar vibrator.
In FIG. 1 there is shown an example of a conventional zinc oxide thin film tuning fork vibrator which includes a main body 1 having leg portions 2 and 3. Zinc oxide thin films 4 and 5 are formed on the respective side walls 2a and 3a of leg portions 2 and 3 and Cr-Au or Ti-Au base electrodes 6 and 7 are formed, respectively, on top of the zinc oxide thin films 4 and 5. Zinc oxide thin films 4 and 5 can be formed by a chemical vapor deposition or a physical vapor deposition process such as vacuum deposition, sputtering, or ion plating.
More specifically, Cr-Au or Ti-Au base electrodes 6 and 7 have the structure shown in FIG. 2 and comprise an inner electrode layer 8 of Cr or Ti and an outer electrode layer 9 made of Au. Cr or Ti are used for their adhesion properties and Au is used for its wire bonding or soldering characteristics. An example of a ZnO thin film tuning fork resonator having a gold electrode is described in "Tuning Fork Resonators for Electronic Wrist Watches Using ZnO Sputtered Film", S. Fujishima et al., Proceedings of the First Meeting on Ferroelectric Materials and their Applications, Kyoto, Japan (1977), pp. 49-52. Cr-Au base electrodes include, as well as a Cr-Au electrode, a Cr-Pt-Au electrode, a Cr-Pd-Au electrode, a Cr-Ni-Au electrode, a Cr-Cu-Ni-Au electrode, and a Cr-Cu-Au electrode. Similarly, Ti-Au base electrodes include, as well as a Ti-Au electrode, a Ti-Cu-Au electrode, a Ti-Pt-Au electrode, and a Ti-Pd-Au electrode. In the case of a Ti-Au base or Cr-Au base electrode, the Ti or Cr layer has a film thickness ranging between 200 .ANG. and 500 .ANG. and the Au layer has a film thickness ranging between 3,000 .ANG. and 5,000 .ANG.. They can be formed by techniques such as resistance heating deposition, electron beam heating deposition or ion plating.
These electrodes for zinc oxide thin film, however, have the following defects. Since Ti or Cr in the first electrode layer has a high affinity, the Ti or Cr diffuses into the zinc oxide film or receives oxygen form the zinc oxide which deteriorates the electrical characteristics of the zinc oxide thin film and can seriously change its resonance frequency. This phenomenon is accelerated when the electrode is subjected to a high temperature life test, resulting in a further deterioration in the electrical characteristics of the zine oxide thin film.
An Al electrode structure for a zinc oxide thin film is also known. It has been used because it is inexpensive and can be easily bonded. The Al electrode, however, also has a high affinity, diffusing into the zinc oxide thin film and causing a deterioration in the film's electrical characteristics. The diffusion of 3-valent A1 in zinc oxide, a 2-valent semiconductor, deteriorates the electrical characteristics of the zinc oxide thin film and seriously changes its resonance frequency. When the A1 electrode is subjected to a high temperature life test, the change is accelerated and the deterioration of the electrical characteristics is further increased as compared with the Ti-Au base or Cr-Au base electrodes.
It has therefore been desirable to develop an improved electrode structure for zinc oxide thin films.